High Voltage Direct Current (HVDC) electric power transmission systems use direct current for bulk transmission of electrical power, in contrast to the more common Alternating Current (AC) systems. For long-distance transmission, HVDC systems may be less expensive and suffer lower electrical losses. Long distance point-to-point HVDC transmission systems generally have lower overall investment cost and lower power losses than equivalent AC transmission systems. HVDC conversion equipment at the terminal stations may be costly, but the total HVDC transmission line costs over long distances are lower than an AC line of the same distance. Further, HVDC requires less conductor per unit distance than an AC line, as there is no need to support three phases as is typical of an AC transmission line and, also unlike an AC transmission line, for HVDC transmission lines there is no skin effect (e.g., the tendency of AC current to become distributed within a conductor such that the current density is largest near the surface of the conductor, and decreasing with greater depths into the conductor). For underwater power cables, HVDC avoids the heavy currents required to charge and discharge the cable capacitance of each AC cycle.
HVDC transmission may also be selected for other technical benefits. HVDC allows power transmission between unsynchronized AC transmission systems. Since the power flow through an HVDC link can be controlled independently of the phase angle between source and load, it can stabilize a network against disturbances due to rapid changes in power generation/consumption. HVDC also allows transfer of power between grid systems running at different frequencies, such as 50 Hz and 60 Hz AC systems. This improves the stability and economy of each network, by allowing exchange of power between incompatible networks. Moreover, HVDC power flow between separate AC systems may be automatically controlled to support either network during transient conditions, but without the risk that a major power system collapse in one network will lead to a collapse in the second network. HVDC improves power system controllability. With at least one HVDC link embedded in an AC grid, the controllability feature may be particularly useful where control of energy trading is desired.
While for shorter distances the higher cost of DC conversion equipment compared to an AC system may still be justify the use of an AC transmission system, due to the benefits of HVDC transmission, the addition of HVDC transmission to the electric grid may help create a less costly and more stable overall electric power system.